1. Field of the Invention
The present invention relates to an effective technique to be applied to a mobile communication system in which congestion may occur, a device used in the system, and a method executed in the device.
2. Description of the Related Art
Cellular phones have been widely used and have attracted attentions as a system used to make sure the safety of peoples in a disaster area when disaster or the like occurs. FIG. 14 is a diagram showing a configuration example of a mobile communication system. Mobile terminals P1 communicate with a radio base station P2 by using radio channels. The radio base station P2 is connected to a radio base station control device P3 and controlled by the radio base station control device P3. The radio base station control device P3 is connected to a core network P4, whereas the core network P4 is connected to a fixed telephone P6 via a fixed telephone network P5.
The radio base station P2 can use only limited given radio resources. The radio channels can be classified into common channels and dedicated channels according to intended uses thereof. Further, the radio channels can be classified into uplink channels and downlink channels according to the communication directions. The limited radio resources are divided and allocated to uplink common channels, uplink dedicated channels, downlink common channels, and downlink dedicated channels. The dedicated channels are special channels, each of which is to be allocated to each mobile terminal when necessary. Specific examples of the downlink common channels include a report channel and a paging channel. Specific example of the uplink common channel includes a random access channel.
FIG. 15 is a diagram showing a procedure example performed when the mobile terminal P1 makes a call to the fixed telephone P6. Described above of horizontally-extending arrows shown in FIG. 15 are information, instruction, or processing to be conveyed, respectively, from the start to the end of the arrow. In FIG. 15, a channel to be used to send the information is described in parentheses.
First, the radio base station control device P3 sends, via the radio base station P2, information on the radio base station P2 (report information) and the like to all the mobile terminals P1 (in FIG. 14, the two mobile terminals P1) by using the report channel, which is one of the downlink common channels (PS1). In an off-hook state, the mobile terminal P1 requests a dedicated channel to perform a call connection procedure based on the received reporting information by using the random access channel, which is one of the uplink common channels (PS2). When there are still sufficient radio resources left, the radio base station control device P3 sends information on dedicated channel allocation, to the mobile terminal P1 via the radio base station P2 by using a downlink common channel (PS3). Upon reception of the information, the mobile terminal P1 uses a dedicated channel which has been allocated, to send a call connection request to the radio base station control device P3 (PS4). Then, the mobile terminal P1 performs a call connection establishment procedure (call connection establishment sequence) (PS5) to allow a phone call to be connected between the mobile terminal P1 and the fixed telephone P6 (PS6).
FIG. 16 is a diagram showing a procedure example performed when the fixed telephone P6 makes a call to the mobile terminal P1, which is contrary to the case of FIG. 15. The mobile terminal P1 receives report information from the radio base station P2 and the radio base station control device P3 as in the case of FIG. 15 (PS1). The fixed telephone P6 first issues a call connection request (PS7). When the call connection request is received by the core network P4 via the fixed telephone network P5, the core network P4 instructs appropriate radio base station P2 and radio base station control device P3 to page the mobile terminal P1 (PS8). Upon reception of the instruction, the radio base station P2 and the radio base station control device P3 page the mobile terminal P1 by using the paging channel, which is one of the downlink common channels (PS9) The mobile terminal P1, which has been paged, requests a dedicated channel to perform a call connection procedure, by using the random access channel (PS10). If there are radio resources left, the radio base station P2 and the radio base station control device P3 send information on dedicated channel allocation, to the mobile terminal P1 by using a downlink common channel (PS11). Upon reception of the information, the mobile terminal P1 uses a dedicated channel which has been allocated, to send a call connection request to the radio base station control device P3 (PS12). Then, the mobile terminal P1 performs a call connection establishment procedure (call connection establishment sequence) (PS13) to allow a phone call to be connected between the mobile terminal P1 and the fixed telephone P6 (PS14).
Patent Document 1: JP 11-178073 A
Patent Document 2: JP 2000-253460 A
However, in the conventional system described above, a problem occurs when an excessive congestion occurs at a time of disaster. Specifically, at a time of disaster, the users of mobile terminals in the disaster area may simultaneously make calls to contact their families and acquaintances, and their families and acquaintances may also simultaneously make calls to the users of mobile terminals in the disaster area to make sure of the safety of them. In this case, an excessive congestion occurs in mobile communication networks (such as cellular phone networks and PHS networks) of the disaster area, leading to a situation in which the calls are not connected.
As a technology for solving the problem, a radio connection system has been proposed, in which urgent information of a subscriber is notified to the call destination even when all radio communication channels are being used (see Patent Document 1). In this system, when a phone call cannot be connected between the subscriber and a center device, a base station sends, to the center device, information indicating that this is an urgent call from an important subscriber. However, in the system, the center device is just notified that the urgent call has been made from the particular subscriber, but the subscriber cannot send desired information to the center device. In addition, the subscriber cannot send information to desired users (such as relatives and friends of the subscriber) either.
For making calls to emergency numbers, such as 110 and 119, special radio resources are reserved. Therefore, even when a general congestion as described above has occurred, calls to those numbers can be connected. Another technology has been proposed in which, while all communication slots of radio channels are being used, when an urgent call is additionally made from a mobile terminal, a radio base station allocates a slot which has been used as a control slot, to a communication slot for the urgent call, to allow the urgent call to be connected to the police or the fire department (see Patent Document 2). However, when outgoing and incoming calls use up the resources, it becomes impossible to make a call in the same way as with other conventional technologies. Thus, as in Patent Document 1, the user cannot send desired information to a desired user.